There is known the need to install devices for detecting radionuclides, for example, in ports, airports, and stations, and in industrial plants or plants of other kinds that treat radioactive elements or make use thereof or that have the need to carry out monitoring for the presence of radioactive material and prevent introduction of radioactive material.
In particular, above all for security requirements and in response to possible terrorist threats, devices known as “radiation portal monitors” (RPMs), or simply “portals”, are increasingly widespread.
These devices must meet international standards, such as, for example, those set down by the following standards:                IEC 62244 (2006): “Radiation protection instrumentation—Installed radiation monitors for the detection of radioactive and special nuclear materials at national borders”; and        ANSI N42.35 (2006): “American National Standard for Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security”.        
A review of detection devices used in portals may be found in the following technical and scientific papers:    R. Kouzes, et al., “Detecting Illicit Nuclear Materials”, American Scientist 93 (2005) 244.    R. Kouzes, et al., “Passive neutron detection for interdiction of nuclear material at borders”, Nucl. Instr. and Meth. A583 (2008) 383.    P. E. Fehlau, C. Garcia Jr., R. A. Payne, E. R. Shunk, “Vehicle monitors for domestic perimeter safeguards”, Los Alamos National Laboratory, LA-9633-MS UC-15, January 1983.
The majority of the devices currently available for providing portals is typically made up of two distinct active parts: one part is dedicated to detecting gamma radiation, and the other to detecting neutrons.
Gamma radiation is typically detected via devices that use plastic scintillators (for example, polyvinyltoluene) or, in some cases and especially where higher spectrometric performance is required, crystalline scintillators (for example, made of sodium iodide) or semiconductor detectors (for example, made of germanium).
Neutron radiation is instead typically detected via the use of multiple detectors (frequently having a tubular shape) using 3He in the gaseous state. The increase in cost of said raw material, due to the reduction of its availability starting from the years 2000, renders, however, necessary research and development of alternative solutions. In fact, 3He, which is rare in nature, is mainly generated, artificially, as by-product of the process of production of nuclear warheads. Two concomitant causes have led in the last decades to a reduction in the availability of 3He at a global level: on the one hand, the process of disarmament on the part of the main Western Countries starting from the last decades of the last century, and, on the other hand, the simultaneous increase in the demand for new installations of portals at national borders in order to increase security of citizens, following upon the acts of terrorism in the early 2000s. The reduced availability of 3He has in turn caused an increase in the cost of the raw material, which represents the current problem of this category of devices.
There hence arises the need to identify materials alternative to 3He for neutron detection.
Neutron detectors currently under study for use in portals instead of detectors based upon 3He are described in the following paper:    Richard T. Kouzes, James H. Ely, Luke E. Erikson, Warnick J. Kernan, Azaree T. Lintereur, Edward R. Siciliano, Daniel L. Stephens, David C. Stromswold, Renee M. Van Ginhoven, Mitchell L. Woodring: “Neutron detection alternatives to 3He for national security applications”, Nucl. Instr. Meth., A623 (2010), pp. 1035-1045.
The solutions described in the above paper, as others available today or under development (which, for example, resort to scintillators with lithium and boron trifluoride in the gaseous state, or based upon excimers of noble gases or semiconductors with boron) are not, however, fully satisfactory, above all because in some cases they manifest a lower detection efficiency in regard to 3He and, in other cases, because of the toxicity and danger of the materials used.
Other devices under development are described in the international patent application No. WO2013116241-A1. In particular, this document discloses devices for detecting neutrons and gamma rays constituted by two types of different and distinct detectors, one for the neutrons and one for the gamma rays. The two types of detectors use different active elements, such as, for example, two types of different scintillators (for example, a plastic scintillator for gamma rays, and a zinc-sulphide-based composite scintillator for neutrons), or a scintillator and an ionization chamber.
In certain embodiments described in WO2013116241-A1, for detection of neutrons a composite scintillator is used, which contains, in addition to the scintillator element, also Cd or Gd; these elements are used in (solid or liquid) mixture with the scintillator.
In some embodiments, the neutron and gamma-ray detectors are “read” by a common sensor (photomultiplier), and discrimination between neutrons and gamma rays is obtained from analysis of the time plot of the signal.
Also these devices seem to present limits, above all in terms of simplicity of construction and effectiveness and reliability of operation, for example in so far as they require use and management of different active elements, some of which are also relatively complex to produce. Also the system of data analysis for discrimination of gamma rays and neutrons, based upon the time plot of the signal received, may not always prove satisfactory.
A device that is able to detect both neutrons and photons using a single active element for both types of radiation is described in the following paper:    De Vita R., et al., “A large surface neutron and photon detector for civil security applications”, Nuclear Instruments and Methods in Physics Research, A617 (2010), pp. 219-222.
The device described therein is based upon a detector formed by an array of plastic scintillator bars wrapped in sheets of a reflecting material containing gadolinium oxide for capturing neutrons. Each bar is coupled to two photomultipliers. This device, both as regards the geometry of the plastic scintillator elements (bars) and owing to the fact that each bar is coupled to two photomultipliers, is not fully satisfactory.